The present invention is directed to electronic devices for suppressing background noise of the type that, for example, occurs when a mobile-telephone user employs a hands-free telephone in an automobile.
A mobile-cellular-telephone user's voice often has to compete with traffic and similar noise, which tends to reduce the intelligibility of the speech that his cellular telephone set transmits from his location. To reduce this noise, a general type of noise-suppression system has been proposed in which the signal picked up by the microphone (i.e., speech plus noise) is divided into frequency bins, which are subjected to different gains before being added back together to produce the transmitted signal. (Of course, this operation can be performed at the receiving end, but for the sake of simplicity we will describe it only as occurring at the transmitter end.) The different gains are chosen by reference to estimates of the relationship between noise and voice content in the various bins: the greater the noise content in a given bin, the lower the gain will be for that bin. In this way, the speech content of the signal is emphasized at the expense of its noise content.
The noise-power level is estimated in any one of a number of ways, most of which involve employing a speech detector to identify intervals during which no speech is present and measuring the spectral content of the signal during those no-speech intervals.
Properly applied, this use of frequency-dependent gains does increase the intelligibility of the received signal. It nonetheless has certain aspects that tend to be disadvantageous. In the first place, many implementations tend to be afflicted with "flutter." A certain minimum record, or frame, of input signal is required in order to divide it into the requisite number of frequency bands, and the abrupt changes in the gain values at the end of each such record during non-speech intervals can cause a fluttering sound, which users find annoying. Methods exist for alleviating this problem, but they tend to have drawbacks of their own. For instance, some systems temporally "smooth" the gain values between input records by incrementally changing the gains, at each sample time during a frame, toward the gain dictated by the computation at the end of the last frame. This approach does largely eliminate the flutter problem, but it also reduces the system's responsiveness to changing noise conditions.
One could solve the frame problem by using a bank of parallel bandpass filters, each of which continually computes the frequency content of its respective band. But most commonly used bandpass-filter implementations would make obtaining the necessary resolution and reconstructing the gain-adjusted signals prohibitively computation-intensive for many applications.
Another drawback of conventional implementations of this general approach is that they distort the speech signal: the relative amplitudes of the frequency components in the transmitted signal are not the same as they were in the signal that the microphone received.